The present invention relates to a homogeneous bed and catalyst particles with improved bimetallic and bifunctional effects, the catalyst particles having reduced local composition fluctuations, resulting in much improved catalytic performances, in particular as regards activity and gasoline yields. Such a bed is termed xe2x80x9chomogeneous on a micronic scalexe2x80x9d. Such particles can even be termed xe2x80x9chomogeneous on a nanometric scalexe2x80x9d. The invention also relates to a process for transforming hydrocarbons into aromatic compounds using that catalyst, such as a gasoline reforming process and a process for producing aromatic compounds.
Catalysts for gasoline reforming and/or for aromatic compound production are well known. They generally contain a matrix, at least one noble metal from the platinum family, at least one halogen and at least one promoter metal, also known as an additional metal.
Of the promoter metals, tin in particular is used for regenerative processes and rhenium is used for fixed bed processes.
Catalysts for gasoline reforming and/or for aromatic compound production are bifunctional catalysts having two functions which are essential for producing the correct performances: a hydro-dehydrogenating function which dehydrogenates naphthenes and hydrogenates coke precursors, and an acid function which isomerises the naphthenes and paraffins and cyclises long paraffins. The hydro-dehydrogenating function can be provided by an oxide such as molybdenum oxide MoO3, chromium oxide Cr2O3 or gallium oxide Ga2O3, or by a metal from column 10 (Ni, Pd, Pt). Metals, in particular platinum, are known to be much more active than oxide phases for hydro-dehydrogenating reactions, and for this reason metallic catalysts have replaced supported oxide catalysts when reforming gasoline and/or producing aromatic compounds. However, metals such as Ni, and to a lesser extent palladium and platinum, also exhibit a hydrogenolysing activity, to the detriment of the desired gasoline yields when reforming gasoline and/or when producing aromatic compounds. This hydrogenolysing activity can be substantially reduced, and thus the catalyst selectivity can be increased, by adding a second metal such as tin. Further, adding a second metal such as iridium or rhenium increases the hydrogenating properties of the platinum, encouraging hydrogenation of coke precursors and thus increasing the catalyst stability. These various reasons have encouraged the success of bimetallic catalysts over first generation monometallic catalysts. More recently, trimetallic catalysts have been introduced, which retain the increased stability of bimetallic catalysts while increasing the gasoline selectivities of such catalysts.
Selectivity can be increased by various means. In the prior art, U.S. Pat. No. 5,128,300 recommends, for catalyst extrudates, a homogeneous distribution of tin with a local composition fluctuation of no better than 25% about the average tin content, that being 0.1-2% by weight of the catalyst.
We have discovered, and this constitutes the subject matter of the present invention, that catalyst performances could be substantially improved not only by limiting the variation of a single element, but by controlling the relative fluctuations of the ratio of the concentrations (compositions) of noble metal (platinum) to the additional metal and/or of the concentrations (compositions) of noble metal (platinum) to the halogen. Thus homogeneity of the bimetallic noble metalxe2x80x94additional metal effect and/or the bifunctional noble metal-acid effect is obtained in the particle bed which improves the overall performances of the process in which this catalyst is used.
More precisely, the invention is concerned with a catalyst comprising at least one amorphous matrix, at least one noble metal, at least one additional metal M and at least one halogen, and in which, for one catalyst particle, CPt is the local concentration of noble metal, CM is the local concentration of additional metal M, and Cx is the local concentration of halogen, in which catalyst in the form of a homogeneous catalyst particle bed the local dispersion of the value of CPt/CM or CPt/Cx is termed homogeneous, which corresponds to at least 70% of the values CPt/CM or CPt/Cx for the catalyst particle bed deviating by a maximum of 30% from the local average ratio.
The amorphous catalyst matrix is generally a refractory oxide such as magnesium, titanium or zirconium oxide, alumina or silica, used alone or mixed together. The preferred support contains alumina or it is alumina.
For gasoline reforming reactions and/or aromatic compound production reactions, the preferred matrix is alumina, and advantageously the specific surface area is 50-600 m2/g, preferably 150-400 m2/g.
The catalyst also contains at least one noble metal from the platinum family (Pt, Pd, Rh, Ir), preferably platinum. The catalyst can advantageously contain a noble metal (such as Pt) and also iridium.
The additional metal M is selected from the group formed by tin, germanium, lead, gallium, indium, thallium, rhenium, manganese, chromium, molybdenum and tungsten. In the case of processes for reforming gasoline and/or for producing regenerative aromatic compounds in a moving bed, the preferred metal is tin, and very advantageously it is associated with platinum (catalysts containing Pt, Sn) and more advantageously, the catalyst further contains tungsten (catalysts containing Pt, Sn, W).
In fixed bed processes, the preferred metal is rhenium; very advantageously it is combined with platinum (catalysts containing Pt, Re); more advantageously still, the catalyst contains indium (catalysts containing Pt, Re, In); further, tungsten can be present (catalysts containing Pt, Re, W or Pt, Re, In, W).
The halogen is selected from the group formed by fluorine, chlorine, bromine and iodine. Chlorine is preferred.
The catalyst generally contains 0.01% to 2% by weight of a noble metal, 0.1% to 15% of a halogen and 0.005% to 10% of an additional metal. Preferably, the catalyst also contains at most 2% of additional metal M, and very advantageously better than 0.1% of that metal. Under these preferred conditions, the catalyst will perform better due to the optimised bimetallic effect.
It should also be noted that the catalyst used in gasoline reforming and/or aromatic compound production processes preferably contains practically no alkali.
The catalyst is in the form of a bed in the form of particles which may be beads, extrudates, three-lobed particles or any other routinely used form.
CPt is the local concentration of noble metal (expressed in % by weight) (the noble metal not necessarily being platinum), CM is the local concentration (by weight) of the additional metal and Cx is the local concentration (by weight) of halogen.
The concentrations can also be expressed in atomic %, as the relative fluctuations will be the same.
The overall composition of the catalyst can be determined by X ray fluorescence carried out on the powdered catalyst or by atomic absorption after acid attack of the catalyst.
In contrast to the overall composition of the catalyst, the local composition on the micronic scale can be measured using an electronic microprobe and can if necessary be complemented by STEM (scanning transmission electron microscopy). This measurement can be made by determining the platinum and additional metal contents in some zones of a few cubic microns along the diameter of a catalyst particle, termed the measurement units. This measurement enables the macroscopic distribution of the metals inside the particles to be determined.
The analyses are carried out using a JEOL JXA8800 electronic microprobe (preferred apparatus) or if necessary using a CAMEBAX type Microbeam, each provided with four wavelength dispersion spectrometers. The acquisition parameters were as follows: acceleration voltage 20 kV, current 30 nA, Pt Mxcex1, Sn Lxcex1, Cl Kxcex1 lines, and count time 20 s or 40 s depending on the level of concentration. The particles (in the figures they were beads) were coated with resin then polished down to their diameter.
It should be noted that the designation xe2x80x9cdiameterxe2x80x9d does not refer only to a bead or extrudate shape, but more generally to any particle shape; the term xe2x80x9cdiameterxe2x80x9d is used to designate the representative length of the particle on which the measurement is made.
The measurements are made on a representative sample of the bed or catalyst batch which will be used for a catalytic bed. It was considered that the analyses ought to be made on at least 5 particles with at least 30 measurements per particle, uniformly distributed along the diameter.
CPt denotes the local concentration of noble metal (expressed as % by weight) (the noble metal not necessarily being platinum), CM denotes the local concentration (by weight) of the additional metal, and Cx denotes the local concentration (by weight) of halogen.
The concentrations could also be expressed in atomic %, the relative fluctuations being the same.
On the basis of the local measurements of CPt, CM and Cx (measurements corresponding to a specific position on the diameter of a particle), the local CPt/CM and/or CPt/Cx ratios can be calculated.
For each radial position, an average local ratio [CPt/CM]m and/or [CPt/Cx]m is calculated (average of the local ratios corresponding to different particles).
Thus the absolute values of the differences between each ratio CPt/CM measured locally and the corresponding average local ratio [CPt/CM]m can be determined. These values are termed xe2x80x9clocal dispersionsxe2x80x9d.
In accordance with the invention, said dispersion is termed homogeneous, meaning that at least 70%, preferably at least 80%, of the values of CPt/CM or CPt/Cx for the catalyst particle bed deviate by a maximum of 30% from the average local ratio.
It is thus said that the local dispersion corresponds for at least 70% of the particles to a confidence interval better than 30%. Preferably, this criterium of homogeneity of the local dispersions is reduced to 30%, preferably 20%, advantageously 15%, even 10%, and even 7% or 5% (that is to say that the values deviate by a maximum of 20% etc . . . )
Thus at any point in the catalyst, a variation in the amount of element M is accompanied by a controlled variation in the platinum content, such that the ratio Pt/M remains within an optimum spread. This approach enables the xe2x80x9cbimetallic effectxe2x80x9d to be fully expressed.
The bimetallic effect corresponds to the quality of the interaction between the platinum and the metal M, which effect conditions the performance level of the catalyst.
An optimum ratio CPt/CM frequently exists (atomic ratio or % by weight) to one side of which the xe2x80x9cbimetallic effectxe2x80x9d is little pronounced and beyond which the activity of the catalyst is reduced by an excess of additional metal. Such an optimum is also observed in trimetallic catalysts between the noble metal and metal M. To fully benefit from the bimetallic effect resulting from adding one or more additional metals, it is important that the CPPt/CM, ratios determined locally for each catalyst particle, varies as little as possible about this optimum value.
A further very important parameter for the catalytic performance of catalysts, in particular those used for gasoline reforming and/or for aromatic compound production, is the amount of halogen (chlorine), in particular the local halogen concentration with respect to the local concentration of noble metal. In this case it is a bifunctional metal-acid effect.
The halogen (chlorine) is responsible for the acid function of catalysts which undertake isomerisation and cyclisation of C6-C11 paraffins. For each catalyst, there is an optimum halogen (chlorine) content. For chlorine contents below this optimum content, catalysts suffer from a lack of activity in particular as regards dehydrocyclisation of P7-P9 paraffins. For chlorine contents over this optimum content, the catalysts exhibit an excessive cracking activity resulting in a large production of C3-C4 fuel gas, and thus a drop in gasoline yields. The optimum chlorine concentration depends on the nature of the support, on its specific surface area and on its structure. It is usually close to 1.0% by weight in commercial catalysts but can be significantly above or below that value for certain particular supports, or in the presence of doping elements such as silicon included in the support.
This results in local CPt/Cx concentration ratios which are significantly different from the local average ratio, resulting in mediocre catalytic performances.
Normally, the local ratio CPt/CM or the local ratio CPt/Cx is constant along the diameter of the catalyst particle. The profile CPt/CM as a function of diameter is thus a xe2x80x9cflat profilexe2x80x9d, like a profile of CPt , CM or Cx with diameter (depending on the case). The noble metal and/or metal M and/or halogen is uniformly distributed in the particle.
For a given particle (preferably, bead), it is possible to determine the absolute values of the differences between each ratio CPt/Cx determined locally and the [CPt/CM]P or [CPt/Cx]p average ratio, respectively, in the particle. These values are termed xe2x80x9cthe radial dispersion in a particlexe2x80x9d.
According to the invention, said dispersion is termed homogeneous on each particle, which means that at least 70% of the values, preferably 80%, deviate by a maximum of 30% from the average value in the particle.
Preferably, this radial dispersion is reduced to 30%, preferably to 20%, advantageously to 15%, even 10%, and even to 7%, or, better, 5%.
In the same way as before, it is thus said that the radial dispersion corresponds for at least 70% of the particles to a confidence interval of better than 30%.
For a given catalyst batch (e.g. for good representation, at least 5 particles, at least 30 measurements per particle) it is possible to determine absolute values of the differences between each ratio CPt/CM or CPt/Cx determined locally and the [CPt/CM]L, or, respectively, [CPt/Cx]L overall average ratio in the batch (average of all the ratios in all the particles). These values are termed, xe2x80x9cthe overall dispersionxe2x80x9d.
According to the invention, said dispersion is termed homogeneous, which means that at least 70% of the values, preferably 80%, deviate by a maximum of 30% from the average value in the batch (overall average ratio).
Preferably, this overall dispersion is reduced to 30%, preferably to 20%, advantageously to 15% or even 10%, and even 7% or 5%.
In the same way as before, it is thus said that the overall dispersion corresponds, for at least 70% of the particles, to a confidence interval which is better than 30%.
It is also of interest to prepare catalysts with different core and peripheral concentrations CPt, CM or Cx. These catalysts have xe2x80x9cbowlxe2x80x9d or xe2x80x9cdomexe2x80x9d distribution profiles. These catalysts with bowl or dome CM or CPt distributions are of interest in certain applications where the effects of diffusion rates of the reactants or products in the catalyst are exploited.
In that case, the value of the local average ratio [CPt/CM]m varies as a function of the particle diameter. This variation can substantially follow a parabolic curve.
A further distribution type is the xe2x80x9csurface-layerxe2x80x9d distribution where the noble metal and/or metal M are distributed at the surface.
In general, the core/edge ratio of concentrations CPt, CM or Cx at the centre and periphery of the catalyst particles can vary from 0.1 to 3.
In the preferred variation, the catalyst contains at least one metal M and the noble metal (preferably Pt) uniformly distributed in the catalyst particle.
In a further possibility, the catalyst contains at least one metal M uniformly distributed in the whole catalyst, the noble metal being xe2x80x9cbowlxe2x80x9d distributed. In a further variation, at least one metal M is uniformly distributed throughout the catalyst, the noble metal being xe2x80x9csurface-layerxe2x80x9d distributed.
Metal M in the above case is advantageously tin. Preferably, the platinum and tin are bowl distributed.